Bond-wire Transformers are commonly used in a variety of RF circuits in Power RF amplifiers, such as Outphasing, Chireix, and Doherty amplifiers.
FIGS. 1a-1f show different views of a known bond-wire transformer 100. FIG. 1a shows the plan view of bond-wire transformer 100, FIG. 1b shows a 3 dimensional view, FIG. 1c shows a cross-section view, FIG. 1d illustrates the schematic equivalent circuit, and FIG. 1e illustrates the primary circuit 100′ of the bond-wire transformer 100 in plan-view, figure if shows the primary circuit 100′ of the bond-wire transformer in cross section 100. Bond-wire transformer 100 includes a substrate 10, which is typically a semiconductor die. A primary inductor, indicated as L1 in the schematic equivalent circuit may be formed by one or more primary bond-wires 12 and primary on-die transmission lines 20. One end of the primary bond-wire 12 is connected to a first primary terminal 14. The other end of each primary bond-wire 12 is connected to a bond pad 11 which is connected to a primary common bonding line 18 using an inter-metal via 21 as illustrated in FIG. 1f. One end of a primary on-die transmission line 20 may be connected to the primary common bonding line 18. The other end of the primary on-die transmission line is connected to a second primary terminal 16. Consequently each of the primary bond-wires 12 and a respective one of the primary on-die transmission lines 20 are electrically connected in series between the first primary terminal 14 and the second primary terminal 16 and form a loop. Pairs of primary bond-wires 12 and the respective primary on-die transmission lines 20 are electrically connected in parallel between the first primary terminal 14 and the second primary terminal 16.
A secondary inductor, indicated as L2 in the schematic equivalent circuit may be formed by one or more secondary bond-wires 22 and secondary on-die transmission lines 30. One end of the secondary bond-wire 22 is connected to a first secondary terminal 24. The other end of each secondary bond-wire 22 is connected to a secondary common bonding line 28. One end of a secondary on-die transmission line 30 may be connected to the secondary common bonding line 28. The other end of the secondary on-die transmission line is connected to a second secondary terminal 26. Consequently each of the secondary bond-wires 22 and a respective one of the secondary on-die transmission lines 30 are electrically connected in series between the first secondary terminal 24 and the second secondary terminal 26 and form a loop. Pairs of secondary bond-wires 22 and the respective secondary on-die transmission lines 30 are electrically connected in parallel between the first secondary terminal 24 and the second secondary terminal 26.
The sets of primary and secondary bond-wires 12, 22 and on-die transmission lines 20, 30 are inter-digitated. In this configuration, the bond-wires provide the magnetic coupling, while the on-die lines are provided to close the loop.
FIGS. 2a and 2b illustrate the steps for manufacturing the bond-wire transistor 100″. As shown in FIG. 2a, the on-die metal interconnects are formed on substrate 10 using one or more metal layers during the fabrication of the semiconductor device. The first primary terminal 14, the primary on-die transmission line 20, the first secondary terminal 24, and the secondary on-chip transmission line 30 may be formed on a top metal layer and dimensioned such that a bond-wire can be directly attached. The second primary terminal 16, the primary common bonding line 18, second secondary terminal 26, and the secondary common bonding line 28 are formed on a lower metal layer. Inter layer vias (not shown) are used to make the connections between primary on-chip transmission line 20 and the primary common bonding line 18, the primary on-chip transmission line 20 and the second primary terminal 16, the secondary on-chip transmission line 30 and the secondary common bonding line 28 and the secondary on-chip transmission line 30 and the second secondary terminal 26. The bond-wires to form the primary and secondary inductors may be placed and attached with a conventional wire-bonding machine during packaging and assembly of the semiconductor device.
The bond-wire transformer 100 may provide better performance compared to on-die coupled inductors. However, the bond-wire transformer efficiency is still affected for example by Eddy currents induced in the substrate, particularly for low-ohmic substrates which may be used for Laterally Diffused Metal-Oxide-Semiconductor (LDMOS) devices and inefficient coupling between the bond-wires.
The result of these losses may be a drop in efficiency; loss in power; and increased operating temperature; if the transformer is used at the input, loss in gain. These losses may limit the use of bond-wire transformers in high power applications ranging from 1 to 100 Watts.